The present invention is concerned with exhaust gas recirculation in internal combustion engines, and more particularly is concerned with a control system for controlling the quantity of exhaust gas recirculation which is effected during the operation of an internal combustion engine. In particular, the present invention concerns a refinement of a control means of the electronic type for exhaust gas recirculation.
As a method of reducing the concentration of NOx in the exhaust gases of an internal combustion engine, exhaust gas recirculation, which involves recirculating a part of the exhaust gases from the exhaust passage of the engine into the intake passage or intake manifold, is often practiced; and it is a valid way for performing such purification. However, there is a tendency for this exhaust gas recirculation to have a harmful effect on the output power of the engine, and also there is a danger that the fuel economy of the engine, and its drivability, if incorporated in a vehicle, may be deteriorated.
In general, from the point of view of ensuring overall satisfactory performance of an internal combustion engine, from the point of view of power output, fuel consumption, and driveability, the best performance of providing exhaust gas recirculation is one which provides a fixed ratio of exhaust gas which is recirculated, compared to the intake amount of air taken in through the inlet of the carburetor or the like to the engine.
Thus, in order to perform exhaust gas recirculation in the best possible way, an exhaust gas recirculation control means is required, which regulates the amount of exhaust gas recirculated, in response to variation of the operating conditions of the engine.
One such exhaust gas recirculation control means which has already been proposed, and practiced, has an exhaust gas recirculation flow control valve which by its opening and closing operation controls the amount of exhaust gas flowing through an exhaust gas recirculation passage, and is opened or closed in accordance with the amount of vacuum supplied to its vacuum chamber, a changeover valve which selectively connects the vacuum chamber of said control valve to a vacuum source or the atmosphere, and a computing controller which compares the vacuum in said vacuum chamber or valve lift of said control valve with a preset target value and changes over said changeover valve so that the actual vacuum in said vacuum chamber or the valve lift of said control valve is maintained in the close vicinity of the target value.
In such an exhaust gas recirculation control means the computing controller compares the actual value of vacuum existing in the vacuum chamber of, or, alternatively, the current amount of valve lift of, said exhaust gas recirculation flow control valve, with the target value in synchronization with clock pulses, and generates an ON/OFF signal or a pulse signal having a duty ratio which changes in accordance with the result of the comparison, said signal being supplied to said changeover valve. Therefore, the computing controller delivers an ON/OFF signal or a varying duty ratio signal of a frequency which corresponds to the frequency of the clock pulse signal, and the changeover valve is changed over at this frequency.
In this case, if, during the time between one clock pulse signal and the next, the driving condition of the engine changes, so that the target value is changed, then when the next clock pulse signal occurs the comparison between the new target value and the actual value of vacuum in the vacuum chamber, or the amount of valve lift, of said exhaust gas recirculation flow control valve is performed, and according to the result of this comparison the changeover valve is controlled so as to make said actual value come closer to the new target value. However, in the interval before the next clock pulse signal occurs, after the change in the driving condition of the engine, exhaust gas recirculation is performed in an amount suitable to the previous engine driving condition, and therefore, in the meantime, before the next clock pulse signal occurs, exhaust gas recirculation is not performed in an exactly correct amount. Therefore, an approximation error exists in this form of control.
If the frequency of the clock signal is made higher, of course this approximation error is diminished, and therefore the accuracy of the exhaust gas recirculation amount is increased. However, as the frequency of this clock pulse signal is increased, the problem arises that the changeover valve, which has to respond to signals of the same frequency as this clock pulse signal, suffers as regards its durability. Thus in practice, the expedient of increasing the frequency of the clock pulse signal is limited in its application. The frequency practically usable at the present time, with present changeover valves of current design, is only about 10 Hz, approximately.
Further, in this case, there exist a time delay caused by the sluggish movement of the fluid whose pressure is operating in the vacuum chamber of the exhaust gas recirculation flow control valve, before the vacuum therein has been accorded, according to the operation of the changeover valve, to the proper value for providing proper exhaust gas recirculation performance, and a time delay in which the vacuum in the vacuum chamber or the amount of valve lift of the exhaust gas recirculation flow control valve is detected and compared with the target value, and a control signal is given to said exhaust gas recirculation flow control valve and these time delays adversely affect exhaust gas recirculation being performed in an amount suitable to the current driving condition of the vehicle.
In general, the recirculating quantity Q of exhaust gas, in an exhaust gas recirculation system, is determined by the following formula: EQU Q=CA.times.sqrt [(2g/r).times.abs(Pe-Pi)]
Here:
C is a coefficient of flow amount; PA1 A is the passage area in the exhaust gas recirculation passage defined by the exhaust gas recirculation flow control valve; PA1 g is gravitational acceleration; PA1 r is the gas specific gravity; PA1 Pe is the exhaust gas pressure in the exhaust system; and PA1 Pi is the pressure in the intake manifold.
Since the memory of the computing controller remembers target values which represent the passage area A of the exhaust gas recirculation flow control valve which will provide the most desirable amount of exhaust gas recirculation, and which change according to the current value of abs(Pe-Pi), which in turn changes according to the current operating conditions of the engine, if no approximation error, such as due to the aforementioned time delays, existed, then the exhaust gas recirculation flow control valve would always be set up to provide the correct passage area A; but in fact, as such an error always will exist, the passage area which is set up in the exhaust gas recirculation flow control valve is not always correct.
If the error in the passage area A of the exhaust gas recirculation flow control valve is the same, it will be noted that, the greater is the value of abs(Pe-Pi), the greater is the error in the amount Q of exhaust gas recirculation.
Further, since in low load driving, the most desirable amount of exhaust gas recirculation Q is small, the error in the amount Q is more noticeable, in proportion to the actual value of Q. Therefore, in low load and decelerating operation of the engine, the approximation error, which, as explained above, is inevitable, can cause a serious problem with regard to exhaust gas quality and engine performance.
Further, when the exhaust gas recirculation flow control valve is manufactured, in view of cost, to have a valve element formed as a simple cone-shaped element, the smaller valve lift is, i.e. the lower the load of the engine is, the greater is the rate of change of the opening area of the control valve, relative to the lift of the valve element, and therefore, the ratio of the approximation error in the amount of exhaust gas recirculation is greater in low load operation than in high load operation.